(a) Field of the Invention
Broadly speaking, this invention relates to methods and apparatus for discharging static electricity. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for discharging the static charge acquired by a moving vehicle, such as a helicopter.
(b) Discussion of the Prior Art
The accumulation of high electrostatic charges on heavy-lift helicopters and the like creates a very hazardous situation for both personnel and cargo during sling, loading operations when the aircraft, of necessity, is hovering close to the ground. The complete loss of the aircraft and its cargo may occur if the loading operation fails.
It is generally accepted that there are two basic mechanisms involved in the natural charging of a helicopter during flight:
(1) influence or inductive charging caused by the earth's natural electrostatic field with the potential accumulation increasing with altitude, and (2) triboelectric or friction charging caused by contact with dust, sand, rain, hail or snow particles in the atmosphere or by hydrocarbon particles in the engine exhaust gas stream.
Since loading and unloading is typically performed at altitudes between 10 and 100 feet (3 to 30.5 meters), potential and energy accumulations as high as 150,000 volts and 5 joules are respectively encountered. These energy levels are well above the lethal range, and exemplify the critical need for an effective cargo grounding system for the newer series of large helicopters.
Conversely, under high triboelectric or frictional charging conditions caused by dust, sand and dry snow, similar high electric potentials and energies are encountered, necessitating the design of a practical energy discharge system capable of handling these energies without excessive deterioration under adverse operating conditions, thus minimizing the danger to personnel.
It is, thus, a prime object of the invention to reduce the energy levels encountered by cargo handling personnel to well below the 100 millijoule level. This level itself constitutes a severe shock level and commonly occurs in present day small cargo and personnel transport aircraft. Under ideal laboratory conditions, energies of one millijoule are at the in-vitro, sensation threshold level and 10 to 25 millijoule discharges can be felt by most personnel. Under the extremely turbulent downdraft conditions present beneath a hovering helicopter, much higher energies, not exceeding 25 millijoules, can be tolerated by most personnel. The same conditions exist for sensitive cargo or fuels since the violent air-blast in the rotor vortex flow has a disruptive and deadening effect. (NOTE: 25 millijoules in 1.times.10.sup.-9 farads, a typical helicopter capacity at 20 feet, corresponds to 7,070 volts.)
In conjunction with the use of protective clothing and accessories, the prime purpose of the present invention is to prevent any electrical discharge to personnel which would cause them to move involuntarily as a reaction to the shock, in such a manner that the loading operation might be aborted or the cargo handler fall, lose his balance, or otherwise create a condition where he could be injured through falling. Of course, operational personnel are still expected to wear protective clothing consisting of very flexible insulated boots and gloves and coveralls which are impervious to moisture to overcome the psychological hazard of dealing with residual high voltages.
On this basis, a 25 millijoule level would appear adequate to provide a significant reduction in the shocking potential of an aircraft during loading and unloading operations. For a continuous current type discharge through the human body, the sensation threshold level is in the order of 1 mA. Due to the relatively short discharge time, the peak current is higher in a capacitance discharge for the same sensation. Laboratory and field tests have confirmed that when a high resistance is placed in series with the human body, the discharge time of a capacitor increases and the threshold level is then higher than the continuous current sensation threshold, which is in the order of 1 mA. This observation forms the basis for the instant solution to the personnel safety problem using the 1 mA current sensation threshold level.